Almandines

Almandines are the iron-rich member of the garnet group with the chemical formula Fe3Al2Si3O12. They are commonly the dark red to purplish-red garnet used as gemstones; the term almandine is often used to denote gem-grade material of this composition.

Almandine crystallizes in the isometric system and forms euhedral to subhedral crystals, typically dodecahedra or trapezohedra. It has a vitreous luster, a hardness of about 6.5–7.5 on the Mohs scale, and a specific gravity of roughly 3.9–4.3. The gemstone can be transparent to translucent and shows a characteristic deep red color attributable to iron content.

Almandine forms under medium to high-grade metamorphic conditions, particularly in aluminous pelitic rocks during regional metamorphism. It occurs in granulite and schist belts and is commonly found with minerals such as kyanite, sillimanite, and staurolite. Almandine is widely distributed in metamorphic terrains around the world and is a common component of garnet assemblages.

The name almandine derives from almandin, with origins traceable to historic sources; it is commonly linked to the ancient town of Alabanda in Asia Minor, though the exact derivation is uncertain. In addition to gemstones, almandine garnet is used as an abrasive and in various industrial applications.